Expressing transporters on viral envelopes

ABSTRACT

Methods for measuring transporter activity using budding baculoviruses that do not express endogenous transporters on their envelope have a low background level and can measure the target activity with a high sensitivity. Such methods can be used to measure functional changes due to transporter SNPs over a more extensive range of substrates, and can be applied to tailor-made therapies.

TECHNICAL FIELD

The present invention relates to methods for expressing transporters having transporter activity, where the methods comprise using transporter-encoding genes to express transporters on the envelope of budding viruses. In addition, the present invention relates to viruses that express transporters having transporter activity, methods that use these viruses for measuring the transporter activity, and methods of screening for substances that inhibit or promote the transporter activity of the transporters.

BACKGROUND ART

Mammals must take in nutrients from outside the body, and many transporter proteins (transporters) are known to exist in mammalian cells. These transporters mainly act to transport substances essential to the maintenance of life (amino acids, sugars, and such) into cells. In the physiological environment, cells often have multiple transporters that transport the same substrate. In these cases, the individual contribution of transporters to cellular uptake can be estimated using kinetic analysis (calculation of Km, Vmax, and so on; e.g., Wright E. M., Am. J. Physiol. Renal Physiol., 2001, 280: F10-18). Thus, identification of transport substrates and kinetic analysis of transporters are extremely important for revealing their physiological function and their potential in drug delivery.

Currently, methods for analysing transporter function use the following resources as materials: (1) primary cultured cells and cell membrane vesicles (such as lung cells and brush border membrane vesicles) comprising transporters isolated from living bodies; (2) cell lines derived from transporter-comprising cancer cells and so on (such as Caco-2 cells); (3) mammalian cells introduced with transporter genes (such as LLC-PK1 cells and MDCK cells) and Xenopus oocytes; and (4) insect cell membranes (such as Sf9 cell membranes) in which transporters have been expressed using baculovirus expression systems. Of these, mostly used are gene expression systems from mammalian cells and Xenopus oocyte cells. However, even in mammalian and Xenopus oocyte cells introduced with transporter genes, activities from endogenous transporters can be detected, thus elevating background levels (Kanai Y. et al., J. Clin. Invest. 93: 397-404 (1994); Kekuda R. et al., J. Biol. Chem. 271: 18657-18661 (1996); Kekuda R. et al., Am. J. Physiol. 272: G1463-1472 (1997); Yabuuchi H. et al., J. Pharmacol. Exp. Ther. 286: 1391-1396 (1998); Hatanaka T. et al., J. Clin. Invest. 107: 1035-1043 (2001)). For this reason, in some types of transporters, there are reports that describe an activity ratio of only two between cells introduced with genes and those not introduced with genes (parent cell lines). Carrying out kinetic analysis can be problematic in such gene-introduced cells with a low activity ratio.

In Xenopus oocyte cells introduced with transporter genes, transporter activity can be measured using electrophysiological methods. In transporters driven by Na and H ions, and substrates having an electric charge at physiological pH, transporter activity can be detected by measuring the electrical current caused by substrate transport. However, measuring transport activity is difficult when there is no driving force and also when substrates are electrically neutral at physiological pH. Kinetic analysis is also difficult in cases where transporter activity is observed but only a weak current can be detected. In addition, since electrophysiological methods require specific equipment, they are not simple or convenient.

The activity and substrate specificity of transporters that transfer drugs into cells has been reported to influence the drug's bioavailability (for example, Ganaphthy, Leibach, Curr. Biol 3: 695-701 (1991); Nakashima et al., Biochem. Pharm. 33: 3345-3352 (1984); Friedman, Amidon, Pharm. Res. 6:1043-1047 (1989); Okano et al., J. Biol. Chem. 261: 14130-14134 (1986); Muranushi et al., Pharm. Res. 6: 308-312 (1989); Friedman, Amidon, J. Control. Res. 13: 141-146 (1990)). In recent years, research on factors that fluctuate in vivo pharmacokinetics has clarified that drug-metabolising enzymes as well as drug-transporters influence the function of drugs in the body. Known drug-transporters include p-glycoprotein (Annu. Rev. Biochem. 58: 137 (1989)), multidrug resistance protein (Science 258: 1650 (1992); Cancer Res. 55: 102 (1995)), lung resistance protein (Ann. Oncl. 7: 625 (1996); Int. J. Cancer 73: 1021 (1997)), and organic cation transporter (Proc. Natl. Acad. Sci. USA 91: 133 (1994); Molec. Pharmacol. 51: 913 (1997)). Analysis of SNPs is being carried out for these drug-transporters in the same way as for drug-metabolizing enzymes. Transporter SNPs that bring about functional changes have been recently found. These SNPs are receiving attention as one of the factors causing fluctuations between individuals (Ryu S. et al., J. Biol. Chem. 275: 39617-39624 (2000); Tirona R. G. et al., J. Biol. Chem. 276: 35669-35675 (2001)). Currently, functional analysis of transporter SNPs mainly uses mammalian cells introduced with genes. However, this is speculated to be problematic for accurately detecting functional changes caused by SNPs in substrates having a low activity ratio compared to parent cell lines.

DISCLOSURE OF THE INVENTION

The present invention was made considering the above circumstances. An objective of the present invention is to provide methods for measuring the target transporter activity, which have a low background level and a high degree of sensitivity. In addition, another objective of the present invention is to provide methods of screening for substances that inhibit or promote the transport activity of transporters, using the above methods.

Since viruses have no fundamental need to self-reproduce, the present inventors speculated that there was no physiological value in taking up substances essential to maintenance of life. Thus, they focused on the assumption that endogenous transporters may not be expressed (or may be expressed in extremely low amounts) on viral envelopes. The method for measuring transporter activity using budding baculoviruses that do not express endogenous transporters on their envelopes are thought to have a low background level, and to enable a highly sensitive measurement of target activity. Furthermore, by using such methods, functional changes due to transporter SNPs can be measured for a broader range of substrates, and may be applied to tailor-made therapies.

Specifically, the present invention provides:

[1] a method for expressing a transporter having transporter activity, wherein the method comprises culturing a host infected with a recombinant virus that comprises a gene encoding the transporter, and expressing the transporter on the envelope of a budding virus released from the host;

[2] the method of [1], wherein the virus is a baculovirus;

[3] the method of [1] or [2], wherein the transporter is of a non-viral origin;

[4] the method of any of [1] to [3], wherein the transporter is a peptide transporter or an organic anion transporter;

[5] the method of [4] wherein the transporter is PepT1, PepT2, or OATP-C;

[6] a virus that expresses a transporter having transporter activity;

[7] the virus of [6], wherein the transporter is of a non-viral origin;

[8] the virus of [7] wherein the virus is a baculovirus;

[9] the virus of any of [6] to [8] wherein the virus is a budding virus;

[10] the virus of any of [6] to [9] wherein the transporter is a peptide transporter or an organic anion transporter;

[11] the virus of [10] wherein the transporter is PepT1, PepT2, or OATP-C;

[12] a method for measuring the activity of a transporter, wherein the method comprises expressing the transporter on a viral envelope;

[13] the method of [12] wherein the virus is a budding baculovirus;

[14] the method of [12] or [13] wherein the transporter is a peptide transporter or an organic anion transporter;

[15] the method of [14] wherein the transporter is PepT1, PepT2, or OATP-C;

[16] a method of screening for a substance that inhibits or promotes transport activity of a transporter, wherein the method comprises the following steps:

-   -   (a) expressing the transporter on a viral envelope,     -   (b) contacting the transporter with a test substance, and     -   (c) selecting a substance that inhibits or promotes the         transport activity;

[17] the method of [16] wherein the virus is a baculovirus;

[18] the method of [16] or [17] wherein the virus is a budding virus;

[19] the method of any of [16] to [18], wherein the transporter is of a non-viral origin;

[20] the method of any of [16] to [19], wherein the transporter is a peptide transporter or an organic anion transporter;

[21] the method of [20] wherein the transporter is PepT1, PepT2, or OATP-C;

[22] the method of any of [16] to [21], which comprises immobilizing the virus on a support;

[23] the method of [22] wherein the virus is immobilized on the support through an antibody against an envelope protein expressed on the viral envelope; and,

[24] the method of [22] wherein the virus is immobilized on the support through a biotin-streptavidin reaction by biotinylating a protein expressed on the viral envelope.

The present invention relates to methods for expressing transporters having transporter activity, which methods comprise culturing a host infected with a recombinant virus that comprises a gene coding for a transporter, and expressing the transporter on the envelope of a budding virus released from the host. Herein, examples of a “transporter” include peptide transporters, amino acid transporters, and sugar transporters. More specifically, transporters such as those listed in Table 1 can be given as examples.

TABLE 1 Driving force/ Amino Trans- Transporter transport type acids membrane ncbi Reference 4F2hc LAT regulatory 529 1 P08195 Proc. Natl. Acad. Sci. U.S.A. 84 factor (18), 6526-6530 (1987) AE4 Cl/HCO exchange 945 14 AAK16733 Commun. 282 (5), 1103-1109 transport (2001) ATB⁰/ASCT2 Na/neutral amino 541 10 Q15758 J. Biol. Chem. 271 (31), 18657-18661 acid cotransport (1996) ATB⁰⁺ Na/neutral and 642 12 AAD49223 J. Biol. Chem. 274 (34), 23740-23745 basic amino acids (1999) cotransport BAT1/b⁰⁺ Facilitated diffusion 487 12 P82251 Nat. Genet. 23 (1), 52-57 (1999) AT (amino acid) BCRP ATP/primary 655 6 AAC97367 Proc. Natl. Acad. Sci. U.S.A. 95 acive transport (26), 15665-15670 (1998) BSEP ATP/primary 1321 12 AAC77455 Nat. Genet. 20 (3), 233-238 active transport (1998) BTR1 Cl/HCO exchange 891 14 AAK16734 Commun. 282 (5), 1103-1109 transport (2001) CNT1 Na/nucleoside 649 13 NP_004204 Am. J. Physiol. 272 (2), C707-C714 cotransport (1997) CNT2 Na/nucleoside 658 14 O43868 Am. J. Physiol. 273 (6 Pt 2), cotransport F1058-F1065 (1997) CNT3 Na/nucleoside 691 13 NP_071410 J. Biol. Chem. 276 (4), 2914-2927 cotransport (2001) DRA/CLD Cl/HCO exchange 764 P40879 Proc. Natl. Acad. Sci. U.S.A. 90 transport (9), 4166-4170 (1993) EAAC1 Na/acidic amino 525 12 NP_004161 Genomics 20 (2), 335-336 acid cotransport (1994) ENT1 Facilitated diffusion 456 14 NP_004946 Nat. Med. 3 (1), 89-93 (1997) (nucleoside) ENT2 Facilitated diffusion 456 14 AAC39526 Biochem. J. 328 (Pt 3), 739-743 (nucleoside) (1997) FORT Folic acid 591 12 P41440 Commun. 206 (2), 681-687 (1995) GAT1 Na/GABA 599 12 NP_003033 FEBS Lett. 269 (1), 181-184 cotransport (1990) GAT3 Na/GABA 632 12 P48066 Recept. Channels 2 (3), 207-213 cotransport (1994) GLUT1 Facilitated diffusion 492 12 NP_006507 Science 229 (4717), 941-945 (glucose) (1985) GLUT2 Facilitated diffusion 524 12 NP_000331 Proc. Natl. Acad. Sci. U.S.A. 85 (glucose) (15), 5434-5438 (1988) GLUT3 Facilitated diffusion 496 12 NP_008862 J. Biol. Chem. 263, 15245-15248 (glucose) (1988) GLUT4 Facilitated diffusion 509 12 NP_001033 J. Biol. Chem. 264 (14), 7776-7779 (glucose) (1989) GLVR1/PiT-1 Na/Pi 679 10 NP_005406 Cell Growth Differ. 1 (3), 119-127 cotransport (1990) GLVR2/PiT-2 Na/Pi 652 10 NP_006740 J. Virol. 65 (11), 6316-6319 cotransport (1991) LAT1 Facilitated diffusion 507 12 JG0165 Commun. 255 (2), 283-288 (amino acid) (1999) LRP ATP/primary 896 NP_059447 Nat. Med. 1 (6), 578-582 (1995) active transport MCT1 H/organic anion 500 12 NP_003042 Genomics 23 (2), 500-503 cotransport (1994) MCT2 H/organic anion 478 12 O60669 J. Biol. Chem. 273 (44), 28959-28965 cotransport (1998) MCT3 H/organic anion 465 12 O15427 Biochem. J. 329 (Pt 2), 321-328 cotransport (1998) MCT4 H/organic anion 487 12 O15374 Biochem. J. 329 (Pt 2), 321-328 cotransport (1998) MCT5 H/organic anion 505 12 O15375 Biochem. J. 329 (Pt 2), 321-328 cotransport (1998) MCT6 H/organic anion 523 12 O15403 Biochem. J. 329 (Pt 2), 321-328 cotransport (1998) MDR1 ATP/primary 1279 12 AAB69423 Cell 47 (3), 381-389 (1986) active transport MDR3 ATP/primary 1279 12 P21439 EMBO J. 6 (11), 3325-3331 active transport (1987) MRP1 ATP/primary 1531 17 P33527 Science 258 (5088), 1650-1654 active transport (1992) MRP2 ATP/primary 1545 17 Q92887 Cancer Res. 56 (18), 4124-4129 active transport (1996) MRP3 ATP/primary 1527 17 NP_003777 Cancer Res. 57 (16), 3537-3547 active transport (1997) MRP4 ATP/primary 1325 12 NP_005836 Cancer Res. 57 (16), 3537-3547 active transport (1997) MRP5 ATP/primary 1437 12 O15440 Cancer Res. 57 (16), 3537-3547 active transport (1997) MRP6 ATP/primary 1503 17 O95255 Cancer Res. 59 (1), 175-182 active transport (1999) MRP7 ATP/primary 1492 17 Cancer Lett. 162 (2), 181-191 active transport (2001) NaPi-3B Na/Pi 690 8 NP_006415 Commun. 258 (3), 578-582 cotransport (1999) NaSi-1 Na/Si 595 13 NP_071889 Genomics 70 (3), 354-363 cotransport (2000) NHE1 Na/H exchange 815 12 P19634 Cell 56 (2), 271-280 (1989) transport NHE2 Na/H exchange 812 12 NP_003039 Am. J. Physiol. 40 (2), 383-390 transport (1999) NHE3 Na/H exchange 834 12 NP_004165 Am. J. Physiol. 269 (1 Pt 1), transport C198-C206 (1995) NPT1 Na/Pi 467 6-8 Q14916 Genomics 18 (2), 355-359 cotransport (1993) NPT2/NaPi-3 Na/Pi 639 8 NP_003043 Proc. Natl. Acad. Sci. U.S.A. 90, cotransport 5979-5983 (1993) Nramp2/DCT1 Na/Fe 568 12 P49281 Mol. Immunol. 34 (12-13), 839-842 cotransport (1997) NTCP2/ASBT Na/bile acid 348 7 NP000443 J. Biol. Chem. 270 (45), 27228-27234 cotransport (1995) OAT1 Facilitated diffusion 550 12 NP_004781 Commun. 255 (2), 508-514 (organic anion) (1999) OAT2 Facilitated diffusion 548 12 NP_006663 (organic anion) OAT3 Facilitated diffusion 568 12 NP_004781 Commun. 255 (2), 508-514 (organic anion) (1999) OAT4 Facilitated diffusion 550 12 AAK68155 J. Biol. Chem. 275 (6), 4507-4512 (organic anion) (2000) OATP-A Facilitated diffusion 670 12 NP_066580 Gastroenterology 109 (4), 1274-1282 (organic anion) (1995) OATP-B Facilitated diffusion 709 12 NP_009187 Commun. 273 (1), 251-260 (organic anion) (2000) OATP-C Facilitated diffusion 691 12 BAA78639 Commun. 273 (1), 251-260 (organic anion) (2000) OATP-D Facilitated diffusion 710 12 BAA89287 Commun. 273 (1), 251-260 (organic anion) (2000) OATP-E Facilitated diffusion 722 12 BAA89288 Commun. 273 (1), 251-260 (organic anion) (2000) OCT1 Facilitated diffusion 554 12 NP_003048 Mol. Pharmacol. 51 (6), 913-921 (organic cation) (1997) OCT2 Facilitated diffusion 555 12 NP_003049 DNA Cell Biol. 16 (7), 871-881 (organic cation) (1997) OCT3 Facilitated diffusion 551 12 NP_035525 Genomics 55 (2), 209-218 (organic cation) (1999) OCTN1 H/organic 551 11 NP_003050 FEBS Lett. 419 (1), 107-111 cation (1997) OCTN2 Na/organic cation 557 12 O76082 Commun. 246 (3), 589-595 cotransport (1998) PGT Facilitated diffusion 643 12 NP_005612 Commun. 221 (2), 454-458 (organic anion) (1996) rBAT BAT1 regulatory 685 1 AAA81778 J. Biol. Chem. 268 (20), 14842-14849 factor (1993) SDCT1/NaDC-1 Na/dicarboxylic 592 8 NP_003975 Am. J. Physiol. 270 (4 Pt 2), acid cotransport F642-F648 (1996) SGLT1 Na/glucose 664 14 NP00334 Proc. Natl. Acad. Sci. U.S.A. 86 cotransport (15), 5748-5752 (1989) SGLT2 Na/glucose 672 14 NP_003032 Am. J. Physiol. 263 (3 Pt 2), cotransport F459-F465 (1992) SGLT3/SAAT1 Na/glucose 659 14 P31636 J. Biol. Chem. 268 (3), 1509-1512 cotransport (1993) SLC26A6 Cl/HCO exchange 738 11 NP_075062 Genomics 70 (1), 102-112 transport (2000) SVCT1 Na/vitamin C 598 12 NP_005838 Biochim. Biophys. Acta 1461 (1), cotransport 1-9 (1999) UT2 Urea (Facilitated 397 10 Q15849 FEBS Lett. 386 (2-3), 156-160 diffusion) (1996)

Preferable transporters in the present invention are peptide transporters or organic anion transporters, and especially preferable are PepT1, Pept2, and OATP-C. The nucleotide and amino acid sequences of PepT1 and PepT2 are known (human PepT1: GenBankXM_(—)007063, J. Biol. Chem. 270(12): 6456-6463 (1995); human PepT2: GenBank NP_(—)066568, XM_(—)002922, Biochem. Biophys. Acta. 1235:461-466 (1995); mouse PepT1 GenBankAF205540, Biochim. Biophys. Acta. 1492: 145-154 (2000); mouse PepT2: GenBankNM_(—)021301, Biochim. Biophys. Res. Commun. 276: 734-741 (2000)). Furthermore, the nucleotide and amino acid sequence of OATP-C are also known (Table 1: Commun. 273(1), 251-260 (2000)). However, the transporters of the present invention are not particularly limited thereto, as long as they can be expressed on a viral envelope.

Genes encoding the transporters, for example, those listed in Table 1, are registered with the National Centre for Biotechnology Information (NCBI) under the listed accession numbers. For example, based on this sequence information, cDNA libraries or genomic libraries can be screened to obtain genes coding for transporters. More specifically, for example, cDNA or genomic libraries are screened using probes (antibodies against target transporters, or oligonucleotides that hybridise to nucleotide sequences coding for target transporters). Screening can be carried out, for example, by following the standard methods described by Sambrook et al. in Chapters 10 to 12 of “Molecular Cloning: A Laboratory Manual” (New York, Cold Spring Harbor Laboratory Press, 1989). Alternatively, genes encoding target transporters can be isolated using PCR (see e.g., Chapter 14 in the above-mentioned Sambrook et al., 1989).

As methods for expressing transporters on viral envelopes, for example, the method of WO98/46777 or Loisel et al. for expressing envelope proteins using budding baculoviruses can be used (Loisel, T. P. et al., Nature Biotech. 15: 1300-1304 (1997)). More specifically, a recombinant vector for insect cells comprising a gene encoding a transporter is constructed, and inserted, along with baculoviral DNA, into insect cells such as Sf9. The transporter encoded by the recombinant vector is then expressed on mature viral particles (virions), which are released by infected cells to the outside of cells prior to infected cell death. Thus recombinant viruses that express the transporter can be obtained.

In the present invention, a budding virus is a virus that is released from infected cells by budding. Generally, viruses covered by an envelope can bud from cells infected with these viruses, even when the cells have not been destroyed, and are released continuously. On the other hand, adenoviruses that are not covered by an envelope, and herpes viruses that are covered by a nuclear envelope, are released from the cells all at once upon their destruction. In the present invention, budding viruses are particularly preferable. In addition, hosts infected with a recombinant virus in the present invention can be suitably selected by those skilled in the art, depending on the type of virus used, so long as viral replication is possible in the host. For example, insect Sf9 cells can be used when using baculoviruses. Generally, protein expression systems using baculoviruses and insect cells may be useful because modifications such as fatty acid acetylation or glycosylation are carried out at the same time as translation or post-translation, in the same way as in mammalian cells. In addition, the expression level of heterologous proteins in such systems is greater than that in mammalian cell systems (Luckow V. A. and Summers M. D., Virol. 167: 56 (1988)).

The present invention also provides viruses that express transporters comprising transporter activity. Examples of these viruses include baculoviruses, papillomaviruses, polyomaviruses, simian virus 40 (SV40), adenoviruses, Epstein-Bar virus (EBV), and retroviruses. In the present invention, particularly preferable viruses include the AcMNPV (Invitrogen) baculovirus, and budding viruses. In addition, the transporters expressed by the viruses are preferably of a non-viral origin, for example the transporters in Table 1. Of these, peptide transporters and organic anion transporters are preferable, and Pept 1, PepT2, and OATP-C are even more preferable.

The viruses expressing transporters having transporter activity of the present invention can be obtained by, for example, culturing a host that has been infected with a recombinant virus comprising a gene that codes for a transporter. Alternatively, using methods such as the above-mentioned methods of WO98/46777 and Loisel et al (Loisel, T. P. et al., Nature Biotech. 15: 1300-1304 (1997)), a recombinant vector encoding a transporter can be inserted into an insect cell along with a baculovirus, and transporters can be expressed on the envelope of the baculovirus which is released outside of the cell. In addition, using methods like that of Strehlow et al. (D. Strehlow et al., Proc. Natl. Acad. Sci. USA. 97:4209-4214 (2000)), packaging cells such as PA317 can be infected with recombinant Moloney murine leukemia viruses, which are constructed using vectors derived from Moloney viruses introduced with transporter-encoding genes, and the transporters can be expressed on the envelope of the viruses released outside of the cells. However, the viruses of the present invention that express transporters having transporter activity are not limited to those that are constructed using the above methods. They include viruses constructed using any method as long as transporters can be expressed in viral particles or on viral surfaces.

Recombinant viruses constructed as described above can be purified using known methods. For example, known methods for purifying viruses include: augmented density gradient centrifugation (Albrechtsen et al., J. Virological Methods 28: 245-256(1990); Hewish et al., J. Virological Methods 7: 223-228 (1983)), size exclusion chromatography (Hjorth and Mereno-Lopez, J. Virological Methods 5: 151-158 (1982); Crooks et al., J. Chrom. 502: 59-68 (1990); Mento S. J. (Viagene, Inc.) 1994 Williamsburg Bioprocessing Conference), affinity chromatography using monoclonal antibodies, sulphated fucose-containing polysaccharides and the like (Najayou et al., J. Virological Methods 32: 67-77 (1991); Diaco et al., J. Gen. Virol. 67: 345-351 (1986); Fowler, J. Virological Methods 11: 59-74 (1986); TOKUSAIHYOU No. 97/032010 (Unexamined Publication of Japanese National Phase Patent Application)), and DEAE ion exchange chromatography (Haruna et al., Virology 13: 264-267 (1961)). Viruses that express transporters of the present invention are not limited to these, and can be purified using the above methods, or combinations thereof.

The present invention relates to methods for measuring the activity of transporters, which comprise expressing transporters on viral envelopes. For example, measurement of transporter activity using budding baculoviruses can be carried out by the following method. First, if necessary, a substrate to be taken into the virus by the transporters is labelled so as to be detected. For example, the substrate is labelled with radioactive substances, fluorescence, or so on. Next, the substrate is mixed with the budding baculovirus that expresses the transporter, and reacted at 37° C. After a set length of time, the reaction solution is transferred onto a filter such as a cellulose membrane. The substrate taken into the virus is separated by vacuum filtration from the substrate that was not taken up. The filter is washed several times using an ice-cold buffer, and the substrate concentration in the viruses which are trapped on the filter is determined using a liquid scintillation counter, a fluorescence detector, HPLC, or such. Nonspecific uptake can be detected by the substrate uptake into wild type viruses that do not express the transporter. In addition, nonspecific uptake can also be evaluated by carrying out experiments on substrate uptake by coexisting the substrate with transporter inhibitors, or if the substrate is radioactive, by coexisting it with an excess of unlabelled substance. Non-specific uptake can be evaluated by carrying out uptake experiments at 4° C.

As an alternative method, budding baculovirus solutions expressing a transporter can be added to a 96-well plate and incubated overnight at 4° C. to perform plate coating. Alternatively, antibodies against proteins such as gp64 protein, which is highly expressed on viral envelopes, can be added to a 96-well plate, and incubated overnight at 4° C. After this, budding baculoviruses that express the transporter are added to the plate. Antibodies against membrane proteins, such as anti-gp64 antibodies (Novagen, Clontech), can also be used to coat the plate with the viruses. A substrate is then added to the plate, and reaction begins. After a set time, the plate is washed with ice-cold buffer, and substrates that were not taken up by the viruses are removed. The amount of substrates taken up into the virus is measured using a liquid scintillation counter, fluorescence detector, HPLC, or so on. If non-specific adsorption is high, blocking can be carried out prior to measuring activity, using skim milk or such. Non-specific uptake can be detected by substrate uptake into wild-type viruses not expressing the transporter. In addition, transporter inhibitors can be coexisted with the substrate to detect non-specific uptake. Alternatively, when the substrate is a radioactive substance, non-specific uptake can also be evaluated by carrying out uptake experiments by coexisting the substrate with an excess of unlabeled substances. Furthermore, uptake experiments can be carried out at 4° C. to evaluate non-specific uptake.

Usually, cell membrane vesicles prepared from biological resources, cultured cells, and such are preserved in a deep freezer or in liquid nitrogen. However, budding baculoviruses can be preserved at 4° C., and do not require any special freezing devices. In addition, there are no complicated steps such as cell culturing, and there is no requirement for special equipment when measuring activity, as used in electrophysiological methods. Thus, budding baculovirus expression systems are simple methods for measuring transporter activity.

The methods of the present invention for measuring the transporter activity that comprise expressing transporters on viral envelopes can also be applied in searching for substances that inhibit or promote the transporter activity. In particular, methods using budding baculovirus expression systems are simple, and useful in identifying substances that inhibit or promote the transporter activity. Specifically, the methods of the present inventions produce, for example, budding baculoviruses that express target transporters. The radioactive or fluorescent substrates of those transporters are mixed with test substances, and added to the transporter-expressing viruses. Before adding the substrates, compounds can be preloaded to the viruses. Transport activity in the absence of a test substrate is taken as 100, and substances that inhibit or promote the transporter activity are searched for by using changes in activity in the presence of the test substrate as an index. Whether or not the test compound is inhibiting or promoting the transporter activity can be judged by known methods, for example, by labeling the transport target substrate (e.g. peptides in the case of peptide transporters) with a radioactive substance (such as ¹⁴C) or fluorescent substance, and then measuring the amount of that substrate that is taken up by a transporter-expressing virus, etc.

Examples of test substances in the methods of screening for substances that inhibit or promote transport activity of the transporters of the present invention include, but are not limited to, purified or crude proteins (comprising antibodies), gene library expression products, synthetic peptide libraries, cell extracts, cultured cell supernatants, products of fermentation microorganisms, marine organism extracts, vegetable extracts, synthetic low molecular weight compound libraries, peptides, non-peptide compounds, and natural compounds.

Transporters expressed on viral envelopes can be contacted with test compounds in the form of, for example, a purified protein, a form bound to a carrier, a fusion protein with another protein, or a membrane fraction. Herein, examples of carriers on which viruses can be immobilized include synthetic or natural organic high molecular weight compounds, inorganic materials such as glass beads, silica gel, alumina, and active carbon, and these materials coated with polysaccharides or synthetic high molecular weight molecules. Examples of organic high molecular weight compounds comprise a large number of compounds, including polysaccharides such as agarose, cellulose, chitin, chitosan, sepharose, and dextran, polyesters, polyvinyl cholorides, polystyrenes, polysulfones, polyether sulphones, polypropylenes, polyvinyl alcohols, polyamides, silicon resins, fluorocarbon resins, polyurethanes, polyacrylamides, and derivatives thereof. However, so long as the viruses can be immobilized, it is understood that the compositions of the compounds are not especially limited. The form of the carrier is also not particularly limited, and examples include membranes such as a plate, fibers, granules, hollow filaments, nonwoven fabrics, porous forms, and honeycomb forms. However, in the present invention, simplicity of immobilization makes commercially available plates especially preferable. By changing the form, surface area and such of these carriers, the contact area of test compounds can be controlled. Viruses can be immobilized to carriers using, for example, antibodies against the envelope proteins expressed in the viruses. In addition, immobilization onto carriers can also be achieved using streptoavidin, avidin or such when biotinylated beforehand.

The physiological function of transporters can be elucidated by searching for inhibitors or promoters of transporter activity. At the same time, those inhibitors or promoters may be applied to developing pharmaceutical agents for diseases caused by transporter abnormalities.

The present invention's budding baculoviruses that express promoters, and the envelope portions that comprise a transporter of those viruses, can be used as screening antigens or immune antigens when producing transporter antibodies. Preparation of such an antigen can be carried out, for example, according to the methods using baculoviruses described in WO98/46777.

Conventionally, in the construction of anti-transporter antibodies, it was problematic to use an active transporter as an immunogen. However, transporters that are expressed by the methods of the present invention have been confirmed to have transporter activity. Thus, an active transporter can be used as an immunogen by using the present invention's transporter-expressing viruses, or envelope portions that comprise a transporter of those viruses.

Therefore, it is extremely useful to construct antibodies using, as immunogens, the present invention's transporter-expressing viruses and envelope portions that comprise a transporter of those viruses.

Thus, the present invention provides methods for constructing anti-transporter antibodies, which methods comprise using, as immunogens, the present invention's transporter-expressing viruses or envelope portions that comprise a transporter of those viruses. The present invention also provides the antibodies constructed using these methods.

Transporter antibodies of the present invention can be constructed by those skilled in the art, using known methods where non-human animals are administered, by subcutaneous or intraperitoneal injection, several times with transporter-expressing viruses or envelope portions that comprise a transporter of those viruses.

The mammals immunized with sensitizing antigens are not particularly limited, however are preferably selected considering compatibility with parent cells used for cell fusion. Animals generally used include rodents, lagomorphs, and primates.

Examples of rodents that can be used are mice, rats, and hamsters. As lagomorphs, for example, rabbits can be used. Examples of primates are monkeys. Monkeys that can be used include catarrhines (old-world monkeys) such as cynomolgous monkeys, rhesus monkeys, hamadryas, and chimpanzees.

Animals can be immunized with a sensitizing antigen using known methods. General methods include injecting a sensitizing antigen into a mammal by subcutaneous or intraperitoneal injection. Specifically, a sensitizing antigen is diluted with an appropriate volume of Phosphate-Buffered Saline (PBS) or physiological saline, and if desired, the suspension is mixed with an appropriate volume of a conventional adjuvant, for example, Freund's complete adjuvant. After emulsification, this is applied to the mammals. In addition, after this, the sensitizing antigen that has been mixed with an appropriate volume of Freund's incomplete adjuvant is preferably applied every four to 21 days for several times. When immunizing a sensitizing antigen, an appropriate carrier can also be used. Thus immunization occurs, and the increased level of the desired antibody in the serum can be confirmed using conventional methods.

Herein, in obtaining the polyclonal antibodies against the transporters of the present invention, the increase in the level of the desired antibody in the serum is confirmed, and blood is then obtained from the mammals sensitized to the antigens. Serum can be separated from this blood using known methods. As polyclonal antibodies, serum comprising polyclonal antibodies can be used. Where necessary, fractions comprising polyclonal antibodies can be isolated from this serum, and this fraction can also be used. For example, fractions that only recognize the transporters of the present invention can be obtained using affinity columns coupled to these transporters. By purifying these fractions using a protein A or protein G column, immunoglobulin G or M can be prepared.

In obtaining monoclonal antibodies, the increased level of the desired antibody is confirmed in the mammals sensitized to the above antigen, immunocytes can be obtained from the mammals, and then subjected to cell fusion. In this case, immunocytes for cell fusion can preferably be splenocytes. As the parent cells to which the above-mentioned immunocytes are bound, mammal myeloma cells are preferable, and more preferable are myeloma cells that have acquired a characteristic for selection of fusion cells using a pharmaceutical agent.

The above-mentioned cell fusion of immunocytes and myeloma cells can be performed according to known methods, for example, the method of Milstein et al. (Galfre, G. and Milstein, C., Methods Enzymol. (1981) 73, 3-46).

Hybridomas obtained from the cell fusions can be selected by culturing in a conventional selective culture medium, for example HAT culture medium (medium comprising hypoxanthine, aminopterin, and thymidine). Culture in this HAT culture medium is carried out for a continuous period of usually several days to several weeks, a sufficient time to kill cells other than the target hybridomas (non-fusion cells). Next, conventional limiting dilution methods are carried out, and hybridomas that produce the target antibodies are screened and cloned.

In addition to obtaining the above-mentioned hybridomas by immunizing non-human animals with an antigen, human lymphocytes, for example human lymphocytes infected with EB virus, are sensitized in vitro to a virus expressing a transporter of the present invention, or to an envelope portion comprising a transporter of that virus. The sensitized lymphocytes are fused with human-derived myeloma cells that can permanently divide, for example U266. Thus, hybridomas that produce the desired human antibodies that have the activity to bind to the transporters can be obtained (Unexamined Published Japanese Patent Application No. (JP-A) Sho 63-17688).

The obtained hybridomas are transplanted into mice peritoneal cavities, and ascites are recovered from the mice. The monoclonal antibodies thus obtained can be prepared by purification using ammonium sulphate precipitation, protein A or G columns, DEAE ion exchange chromatography, affinity columns to which a transporter of the present invention has been coupled, or the like. In addition to being used for the purification and detection of the transporters of the present invention, the antibodies of the present invention can become candidates for agonists and antagonists of these transporters. Furthermore, these antibodies can also be applied to antibody therapies for diseases involving transporters of the present invention. When using the obtained antibodies for the purpose of application to the human body (antibody therapy), human antibodies and humanized antibodies are preferable due to their low antigenicity.

For example, antibody-producing cells can be obtained by immunizing transgenic animals that comprise a repertoire of human antibody genes, with a virus expressing a transporter that becomes the antigen, or a portion of the viral envelope comprising the transporter. Hybridomas produced by fusing the antibody-producing cells with myeloma cells can be used to obtain human antibodies against the transporter (see International Publication WO92-03918, WO93-2227, WO94-02602, WO94-25585, WO96-33735, and WO96-34096).

In addition to producing antibodies by using hybridomas, immunocytes of antibody-producing sensitized lymphocytes and such that have been immortalized using oncogenes can also be used.

Monoclonal antibodies obtained in this way can also be obtained as recombinant antibodies produced using gene recombination technologies (for example, see Borrebaeck, C. A. K. and Larrick, J. W., Therapeutic Monoclonal Antibodies, UK, Macmillan Publishers Ltd., 1990). Recombinant antibodies can be produced by cloning DNA that encodes them from immunocytes such as hybridomas and antibody-producing sensitized lymphocytes, incorporating into a suitable vector, and introducing this into a host. The present invention also encompasses such recombinant antibodies.

So long as the antibodies of the present invention bind to the polypeptides of the present invention, they can also be antibody fragments, modified antibodies, etc. For example, an antibody fragment can be an Fab, F(ab′)2, Fv, or a single chain Fv (scFv) where Fvs of H chain and L chain are linked by a suitable linker (Huston, J. S. et al., Proc. Natl. Acad. Sci. U.S.A., (1998) 85, 5879-5883). Specifically, the antibody fragments can be produced by treating antibodies with an enzyme such as papain or pepsin. Alternatively, genes encoding these antibody fragments are constructed, inserted into an expression vector, and expressed in appropriate host cells (see for example, Co, M. S. et al., J. Immunol. (1994) 152, 2968-2976; Better, M. and Horwitz, A. H., Methods Enzymol. (1989) 178, 476-496; Pluckthun, A. and Skerra, A., Methods Enzymol. (1989) 178, 497-515; Lamoyi, E., Methods Enzymol. (1986) 121, 652-663; Rousseaux, J. et al., Methods Enzymol. (1986) 121, 663-669; Bird, R. E. and Walker, B. W., Trends Biotechnol. (1991) 9, 132-137).

Antibodies bound to various molecules such as polyethylene glycols (PEG), can also be used as the modified antibodies. “Antibody” in the present invention also encompasses these modified antibodies. Such a modified antibody can be obtained by chemically modifying obtained antibodies. These methods have already been established in the art.

By using known technologies, the antibodies of the present invention can be obtained as chimeric antibodies comprising non-human antibody-derived variable regions and human antibody-derived constant regions, or alternatively, as humanized antibodies comprising non-human antibody-derived complementarity determining regions (CDRs), human antibody-derived framework regions (FRs), and constant regions.

Antibodies obtained as above can be purified until homogenous. The separation and purification of antibodies used in the present invention can use conventional separation and purification methods. For example and without limitation, antibodies can be separated and purified by appropriately selecting and combining chromatography columns such as affinity chromatography columns, filters, ultrafiltration, salt precipitation, dialysis, SDS polyacrylamide gel electrophoresis, isoelectric focusing and so on (Antibodies: A Laboratory Manual. Ed Harlow and David Lane, Cold Spring Harbor Laboratory, 1988). The concentration of the above-obtained antibodies can be determined by measuring absorbance, by enzyme-linked immunosorbent assays (ELISA), etc.

Protein A columns, protein G columns, and such can be used as the columns used for affinity chromatography. For example, as the columns using protein A, Hyper D, POROS, Sepharose F.F. (Pharmacia) and so on can be used.

Examples of chromatography other than affinity chromatography include ion exchange chromatography, hydrophobic chromatography, gel filtration, reverse chromatography, and adsorption chromatography (Strategies for Protein Purification and Characterisation: A Laboratory Course Manual. Ed Daniel R, Marshak et al., Cold Spring Harbor Laboratory Press, 1996). These chromatographies can be carried out using liquid phase chromatography such as HPLC and FPLC.

Examples of the methods for measuring antigen-binding activities of the antibodies of the present invention include absorbance measurements, enzyme-linked immunosorbent assays (ELISA), enzyme immunoassays (EIA), radioimmunoassay (RIA), and immunofluorescence. When using ELISA, the transporters of the present invention are added to a plate to which the antibodies of the present invention have been solid phased. Next, samples comprising a target antibody, for example the culture supernatant of antibody-producing cells or purified antibodies, are added. Secondary antibodies that recognise the antibody, which is labelled with enzymes such as alkaline phosphatase, are then added and the plate is incubated. After washing, an enzyme substrate such as p-nitrophenol phosphate is added, and antigen-binding activity can be evaluated by measuring absorbance. BIAcore (Pharmacia) can be used to evaluate the activity of the antibodies of the present invention.

Transporter-binding antibodies can be screened by ELISA using 96-well plates coated with budding baculoviruses. Antibodies against the viral antigens can be removed by ELISA using wild type viruses as the screening antigen. Alternatively, hybridoma culture supernatant and a wild type virus can be reacted, and after antibodies against the viral antigen have been removed, ELISA can be then carried out using a transporter-expressing virus as the screening antigen to acquire transporter-binding antibodies. Function-inhibiting antibodies can also be screened for from the binding antibodies. In other words, a radioactive or fluorescent substrate of the target transporter can be mixed with a solution comprising antibodies, such as hybridoma culture supernatant, and then added to a transporter-expressing virus. The solution comprising antibodies, such as hybridoma culture supernatant, can be preloaded onto the virus prior to adding the substrate. Transport activity in the absence of antibodies is taken as 100, and function-inhibiting antibodies can be screened for using, as an index, decreased activity in the presence of antibodies. Transporter topography at the cellular level can be revealed by binding antibodies to that transporter. In addition, function-inhibiting antibodies can be added to cell cultures or administered to laboratory animals to make a great contribution to the elucidation of the physiological functions of the transporters. Function-inhibiting antibodies or binding antibodies to transporters associated with disease can be applied as pharmaceutical agents.

The present invention can also be used to evaluate the ways in which transporter activity is altered by changes in amino acid sequence due to mutations, polymorphisms such as SNPs, and so on. For example, many SNPs exist in OATP-Cs, and changes in the amino acid sequence due to these SNPs have been reported (J. Biol. Chem., 276 (2001). By using the methods of the present invention to measure the transport activity of each of these OATP-Cs with altered amino acid sequences, the SNPs that influence transport activity can be identified, transporters with high activity can be screened, and so on.

In addition, after mutants have been created by artificial substitution, insertion, deletion, or addition of transporter amino acid sequences, transporter activity can be measured and transporters with high activity can be screened, or regions that influence transporter activity can be identified. Those skilled in the art can prepare transporters with substituted amino acids by using well-known methods. For example, site-specific mutagenesis and such can be used (Hashimoto-Gotoh, T. et al., Gene, 152, 271-275, (1995); Zoller, M J, and Smith, M., Methods Enzymol, 100, 468-500, (1983); Kramer, W et al., Nucleic Acids Res, 12, 9441-9456, (1984); Kramer, W and Fritz, H J., Method Enzymol, 154, 350-367, (1987); Kunkel, T A., Proc Natl Acad Sci USA, 82, 488-492, (1985); Kunkel, T A., Methods Enzymol, 85, 2763-2766, (1988)).

Further, when using the present invention, substances transported by a transporter can be used as test substances and measure transporter activity to screen for substances that are easily transported by transporters, or substances that are difficult to transport.

The present invention can also be applied to proteins other than transporters. For example, similar methods for measuring activity, screening and such can be carried out for ion channels such as sodium channels, calcium channels, potassium channels, chloride channels, cation channels, and anion channels. In this case, instead of a transporter, a channel is expressed on the viral envelope, and a substance passed through the channel can be used as a substrate. Channels that can be used in the present invention include those listed in Table 2. Thus, the present invention can be used for proteins that can transport or transmit a substance, such as transporters and ion channels (especially proteins which are expressed on membranes and can be transported or passed in a substrate-specific manner).

In addition to the above transporters and ion channels, the present invention can also be applied to G protein coupled receptors (GPCRs).

TABLE 2 Symbol Name Sequence ID ACCN1 amiloride-sensitive cation channel 1, NM_001094 neuronal (degenerin) ACCN2 amiloride-sensitive cation channel 2, NM_001095 neuronal NM_020039 ACCN3 amiloride-sensitive cation channel 3, testis NM_004769 NM_020321 NM_020322 AQP1 aquaporin 1 (channel-forming integral NM_000385 protein, 28 kD) ASIC4 putative acid-sensing ion channel NM_018674 CACNA1A calcium channel, voltage-dependent, P/Q NM_000068 type, alpha 1A subunit NM_023035 CACNA1B calcium channel, voltage-dependent, L type, NM_000718 alpha 1B subunit CACNA1C calcium channel, voltage-dependent, L type, NM_000719 alpha 1C subunit CACNA1D calcium channel, voltage-dependent, L type, NM_000720 alpha 1D subunit CACNA1E calcium channel, voltage-dependent, alpha NM_000721 1E subunit CACNA1F calcium channel, voltage-dependent, alpha NM_005183 1F subunit CACNA1G calcium channel, voltage-dependent, alpha NM_018896 1G subunit CACNA1H calcium channel, voltage-dependent, alpha NM_021098 1H subunit CACNA1I calcium channel, voltage-dependent, alpha 1I NM_021096 subunit CACNA1S calcium channel, voltage-dependent, L type, NM_000069 alpha 1S subunit CACNA2D calcium channel, voltage-dependent, alpha NM_000722 2/delta subunit 1 CACNA2D calcium channel, voltage-dependent, alpha NM_006030 2/delta subunit 2 CACNB1 calcium channel, voltage-dependent, beta 1 NM_000723 subunit CACNB2 calcium channel, voltage-dependent, beta 2 NM_000724 subunit CACNB3 calcium channel, voltage-dependent, beta 3 NM_000725 subunit CACNB4 calcium channel, voltage-dependent, beta 4 NM_000726 subunit CACNG1 calcium channel, voltage-dependent, NM_000727 gamma subunit 1 CACNG2 calcium channel, voltage-dependent, NM_006078 gamma subunit 2 CACNG3 calcium channel, voltage-dependent, NM_006539 gamma subunit 3 CACNG4 calcium channel, voltage-dependent, NM_014405 gamma subunit 4 CACNG5 calcium channel, voltage-dependent, NM_014404 gamma subunit 5 CACNG6 calcium channel, voltage-dependent, NM_031897 gamma subunit 6 CACNG7 calcium channel, voltage-dependent, NM_031896 gamma subunit 7 CACNG8 calcium channel, voltage-dependent, AF288388 gamma subunit 8 CLCA1 chloride channel, calcium activated, family NM_001285 member 1 CLCA2 chloride channel, calcium activated, family NM_006536 member 2 CLCA3 chloride channel, calcium activated, family NM_004921 member 3 CLCA4 chloride channel, calcium activated, family NM_012128 member 4 CLCN1 chloride channel 1, skeletal muscle NM_000083 (Thomsen disease, autosomal dominant) CLCN2 chloride channel 2 NM_004366 CLCN3 chloride channel 3 NM_001829 CLCN4 chloride channel 4 NM_001830 CLCN5 chloride channel 5 (nephrolithiasis 2, NM_000084 X-linked, Dent disease) CLCN6 chloride channel 6 NM_001286 NM_021735 NM_021736 NM_021737 CLCN7 chloride channel 7 NM_001287 CLCNKA chloride channel Ka NM_004070 CLCNKB chloride channel Kb NM_000085 CLIC1 chloride intracellular channel 1 NM_001288 NM_001288 CLIC2 chloride intracellular channel 2 NM_001289 CLIC3 chloride intracellular channel 3 NM_004669 CLIC4 chloride intracellular channel 4 NM_013943 CLIC5 chloride intracellular channel 5 NM_016929 CLIC6 chloride intracellular channel 6 BG184920 CLNS1A chloride channel, nucleotide-sensitive, 1A NM_001293 CNGA1 cyclic nucleotide gated channel alpha 1 NM_000087 CNGA3 cyclic nucleotide gated channel alpha 3 NM_001298 CNGB1 cyclic nucleotide gated channel beta 1 NM_001297 CNGB3 cyclic nucleotide gated channel beta 3 NM_019098 DKFZP43 potassium channel modulatory factor NM_020122 ECAC1 epithelial calcium channel 1 NM_019841 ECAC2 epithelial calcium channel 2 AJ243501 AJ243500 HCN2 hyperpolarization activated cyclic nucleotide- NM_001194 gated potassium channel 2 HCN4 hyperpolarization activated cyclic nucleotide- NM_005477 gated potassium channel 4 HSA24339 voltage-gated sodium channel beta-3 subunit NM_018400 (scn3b gene) HSA27226 calcium channel, voltage-dependent, alpha 2/ NM_018398 delta 3 subunit KCNA1 potassium voltage-gated channel, shaker- NM_000217 related subfamily, member 1 (episodic ataxia with myokymia) KCNA10 potassium voltage-gated channel, shaker- NM_005549 related subfamily, member 10 KCNA2 potassium voltage-gated channel, shaker- NM_004974 related subfamily, member 2 KCNA3 potassium voltage-gated channel, shaker- NM_002232 related subfamily, member 3 KCNA4 potassium voltage-gated channel, shaker- NM_002233 related subfamily, member 4 KCNA5 potassium voltage-gated channel, shaker- NM_002234 related subfamily, member 5 KCNA6 potassium voltage-gated channel, shaker- NM_002235 related subfamily, member 6 KCNA7 potassium voltage-gated channel, shaker- NM_031886 related subfamily, member 7 KCNAB1 potassium voltage-gated channel, shaker- NM_003471 related subfamily, beta member 1 KCNAB2 potassium voltage-gated channel, shaker- NM_003636 related subfamily, beta member 2 KCNAB3 potassium voltage-gated channel, shaker- NM_004732 related subfamily, beta member 3 KCNB1 potassium voltage-gated channel, Shab- NM_004975 related subfamily, member 1 KCNB2 potassium voltage-gated channel, Shab- NM_004770 related subfamily, member 2 KCNC1 potassium voltage-gated channel, Shaw- NM_004976 related subfamily, member 1 KCNC3 potassium voltage-gated channel, Shaw- NM_004977 related subfamily, member 3 KCNC4 potassium voltage-gated channel, Shaw- NM_004978 related subfamily, member 4 KCND1 potassium voltage-gated channel, Shal- NM_004979 related subfamily, member 1 KCND2 potassium voltage-gated channel, Shal- NM_012281 related subfamily, member 2 KCND3 potassium voltage-gated channel, Shal- NM_004980 related subfamily, member 3 KCNE1 potassium voltage-gated channel, Isk-related NM_000219 family, member 1 KCNE1L potassium voltage-gated channel, Isk-related NM_012282 family, member 1-like KCNE2 potassium voltage-gated channel, Isk-related NM_005136 family, member 2 KCNE3 potassium voltage-gated channel, Isk-related NM_005472 family, member 3 KCNF1 potassium voltage-gated channel, subfamily NM_002236 F, member 1 KCNG1 potassium voltage-gated channel, subfamily NM_002237 G, member 1 KCNG2 potassium voltage-gated channel, subfamily NM_012283 G, member 2 KCNH1 potassium voltage-gated channel, subfamily NM_002238 H (eag-related), member 1 KCNH2 potassium voltage-gated channel, subfamily NM_000238 H (eag-related), member 2 KCNH3 potassium voltage-gated channel, subfamily AB033108 H (eag-related), member 3 KCNH4 potassium voltage-gated channel, subfamily NM_012285 H (eag-related), member 4 KCNH5 potassium voltage-gated channel, subfamily U69185 H (eag-related), member 5 KCNIP1 Kv channel-interacting protein 1 NM_014592 KCNIP2 Kv channel-interacting protein 2 NM_014591 KCNJ1 potassium inwardly-rectifying channel, NM_000220 subfamily J, member 1 KCNJ10 potassium inwardly-rectifying channel, NM_002241 subfamily J, member 10 KCNJ11 potassium inwardly-rectifying channel, NM_000525 subfamily J, member 11 KCNJ12 potassium inwardly-rectifying channel, NM_021012 subfamily J, member 12 KCNJ13 potassium inwardly-rectifying channel, AJ007557 subfamily J, member 13 KCNJ14 potassium inwardly-rectifying channel, NM_013348 subfamily J, member 14 KCNJ15 potassium inwardly-rectifying channel, NM_002243 subfamily J, member 15 KCNJ16 potassium inwardly-rectifying channel, NM_018658 subfamily J, member 16 KCNJ2 potassium inwardly-rectifying channel, NM_000891 subfamily J, member 2 KCNJ3 potassium inwardly-rectifying channel, NM_002239 subfamily J, member 3 KCNJ4 potassium inwardly-rectifying channel, NM_004981 subfamily J, member 4 KCNJ5 potassium inwardly-rectifying channel, NM_000890 subfamily J, member 5 KCNJ6 potassium inwardly-rectifying channel, NM_002240 subfamily J, member 6 KCNJ8 potassium inwardly-rectifying channel, NM_004982 subfamily J, member 8 KCNJ9 potassium inwardly-rectifying channel, NM_004983 subfamily J, member 9 KCNJN1 potassium inwardly-rectifying channel, NM_002244 subfamily J, inhibitor 1 KCNK1 potassium channel, subfamily K, member 1 NM_002245 (TWIK-1) KCNK10 potassium channel, subfamily K, member 10 NM_021161 KCNK12 potassium channel, subfamily K, member 12 NM_022055 KCNK13 potassium channel, subfamily K, member 13 NM_022054 KCNK2 potassium channel, subfamily K, member 2 AF004711 (TREK-1) KCNK3 potassium channel, subfamily K, member 3 NM_002246 (TASK-1) KCNK4 potassium inwardly-rectifying channel, sub- NM_016611 family K, member 4 KCNK5 potassium channel, subfamily K, member 5 NM_003740 (TASK-2) KCNK6 potassium channel, subfamily K, member 6 NM_004823 (TWIK-2) KCNK7 potassium channel, subfamily K, member 7 NM_005714 KCNK9 potassium channel, subfamily K, member 9 NM_016601 (TASK-3) KCNMA1 potassium large conductance calcium-acti- NM_002247 vated channel, subfamily M, alpha member 1 KCNMB1 potassium large conductance calcium-acti- NM_004137 vated channel, subfamily M, beta member 1 KCNMB2 potassium large conductance calcium-acti- NM_005832 vated channel, subfamily M, beta member 2 KCNMB3 potassium large conductance calcium-acti- NM_014407 vated channel, subfamily M beta member 3 KCNMB3L potassium large conductance calcium-acti- NM_014406 vated channel, subfamily M, beta member 3-like KCNMB4 potassium large conductance calcium-acti- NM_014505 vated channel, subfamily M, beta member 4 KCNN1 potassium intermediate/small conductance NM_002248 calcium-activated channel, subfamily N, member 1 KCNN2 potassium intermediate/small conductance NM_021614 calcium-activated channel, subfamily N, member 2 KCNN3 potassium intermediate/small conductance NM_002249 calcium-activated channel, subfamily N, member 3 KCNN4 potassium intermediate/small conductance NM_002250 calcium-activated channel, subfamily N, member 4 KCNQ1 potassium voltage-gated channel, KQT-like NM_000218 subfamily, member KCNQ2 potassium voltage-gated channel, KQT-like NM_004518 subfamily, member KCNQ3 potassium voltage-gated channel, KQT-like NM_004519 subfamily, member KCNQ4 potassium voltage-gated channel, KQT-like NM_004700 subfamily, member KCNQ5 potassium voltage-gated channel, KQT-like NM_019842 subfamily, member KCNS1 potassium voltage-gated channel, delayed- NM_002251 rectifier, subfamily S, member 1 KCNS2 potassium voltage-gated channel, delayed- AB032970 rectifier, subfamily S, member 2 KCNS3 potassium voltage-gated channel, delayed- NM_002252 rectifier, subfamily S, member 3 KIAA0439 homolog of yeast ubiquitin-protein ligase AB007899 Rsp5; potential epithelial sodium channel regulator KIAA1169 two-pore channel 1, homolog NM_017901 KV8.1 neuronal potassium channel alpha subunit NM_014379 LOC64181 two pore potassium channel KT3.3 NM_022358 OTRPC4 vanilloid receptor-related osmotically NM_021625 activated channel; OTRPC4 protein P2RX1 purinergic receptor P2X, ligand-gated ion NM_002558 channel, 1 P2RX2 purinergic receptor P2X, ligand-gated ion NM_012226 channel, 2 NM_016318 P2RX3 purinergic receptor P2X, ligand-gated ion NM_002559 channel, 3 P2RX4 purinergic receptor P2X, ligand-gated ion NM_002560 channel, 4 P2RX5 purinergic receptor P2X, ligand-gated ion NM_002561 channel, 5 P2RX7 purinergic receptor P2X, ligand-gated ion NM_002562 channel, 7 SCN10A sodium channel, voltage-gated, type X, alpha NM_006514 polypeptide SCN11A sodium channel, voltage-gated, type XI, AF188679 alpha polypeptide SCN12A sodium channel, voltage-gated, type XII, NM_014139 alpha polypeptide SCN1A sodium channel, voltage-gated, type I, alpha AF225985 polypeptide SCN1B sodium channel, voltage-gated, type I, beta NM_001037 polypeptide SCN2A2 sodium channel, voltage-gated, type II, alpha NM_021007 2 polypeptide SCN2B sodium channel, voltage-gated, type II, beta NM_004588 polypeptide SCN3A sodium channel, voltage-gated, type III, AF225987 alpha polypeptide SCN4A sodium channel, voltage-gated, type IV, NM_000334 alpha polypeptide SCN5A sodium channel, voltage-gated, type V, alpha NM_000335 polypeptide (long (electrocardiographic) QT syndrome 3) SCN6A sodium channel, voltage-gated, type VI, NM_002976 alpha polypeptide SCN8A sodium channel, voltage gated, type VIII, NM_014191 alpha polypeptide SCN9A sodium channel, voltage-gated, type IX, NM_002977 alpha polypeptide SCNN1A sodium channel, nonvoltage-gated 1 alpha NM_001038 SCNN1B sodium channel, nonvoltage-gated 1, beta NM_000336 (Liddle syndrome) SCNN1D sodium channel, nonvoltage-gated 1, delta NM_002978 SCNN1G sodium channel, nonvoltage-gated 1, gamma NM_001039 TALK-1 pancreatic 2P domain potassium channel NM_032115 TALK-1 TASK-4 potassium channel TASK-4; potassium NM_031460 channel TALK-2 TRPC1 transient receptor potential channel 1 NM_003304 TRPC2 transient receptor potential channel 2 X89067 TRPC3 transient receptor potential channel 3 NM_003305 TRPC4 transient receptor potential channel 4 NM_016179 TRPC5 transient receptor potential channel 5 NM_012471 TRPC6 transient receptor potential channel 6 NM_004621 TRPC7 transient receptor potential channel 7 NM_003307 VDAC1 voltage-dependent anion channel 1 NM_003374 VDAC1P voltage-dependent anion channel 1 AJ002428 pseudogene VDAC2 voltage-dependent anion channel 2 NM_003375 VDAC3 voltage-dependent anion channel 3 NM_005662 trp7 putative capacitative calcium channel NM_020389

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing PepT1 activity in PepT1-expressing viruses. The PepT1 activity on the viral envelope was measured as the amount of ¹⁴C glycylsarcosine uptake by the viruses. “Wild type” shows the amount taken up by the wild type virus. “His-PepT1” shows the amount taken up by a PepT1-expressing virus with a His-tag added to the N-terminal.

FIG. 2 is a graph showing PepT2 activity in PepT2-expressing viruses. The PepT2 activity on the viral envelope was measured as the amount of ³H glycylsarcosine uptake by viruses. “Wild type” shows the amount taken up by the wild type virus. “His-PepT2” shows the amount taken up by a PepT2-expressing virus with a His-tag added to the N-terminal.

FIG. 3 is a graph showing OATP-C activity in OATP-C-expressing viruses. The OATP-C activity on the viral envelope was measured as the amount of ³H estrone sulphate conjugate taken up by viruses. “Wild type” shows the amount taken up by the wild type virus. “OATP-C WT” shows the amount taken up by a wild-type OATP-C-expressing virus. “OATP-C N130D” shows the amount taken up by a N130D mutant OATP-C-expressing virus. “OATP-C V147A” shows the amount taken up by a V147A mutant OATP-C-expressing virus. Each of these OATP-Cs comprises a His-tag added to the N-terminal.

FIG. 4 is a graph showing the results of detecting inhibition of the PepT1 activity in PepT1-expressing viruses by an anti-human PepT1 monoclonal antibody. The PepT1 activity on the viral envelope was measured as the amount of ¹⁴C glycylsarcosine taken up by the viruses. Data is shown as the mean+/−SD (n=3-4).

BEST MODE FOR CARRYING OUT THE INVENTION

Herein below, the present invention will be specifically described using Examples, however, it is not to be construed as being limited to thereto.

EXAMPLE 1

1. Preparation of PepT1-expressing Budding Baculoviruses

A full-length PepT1 gene was isolated from a human kidney library using PCR. By inserting the full-length human PepT1 gene into pBlueBacHis2A (Invitrogen), the pBlueBacHis-PepT1 transfer vector was constructed. A Bac-N-Blue transfection kit (Invitrogen) was then used to introduce this transfer vector into Sf9 cells, along with Bac-N-Blue DNA. Thus, a recombinant virus for the expression of human PepT1 was constructed. Specifically, 4 μg of pBlueBacHis-PepT1 was added to Bac-N-Blue DNA, and then 1 mL of Grace's medium (GIBCO) and 20 μL of cell FECTIN reagent was added. This was mixed, incubated for 15 minutes at room temperature, and then added drop-by-drop to 2×10⁶Sf9 cells washed once with the Grace's medium. After incubating for four hours at room temperature, 2 mL of complete medium (Grace's medium which comprises 10% fetal bovine serum (Sigma), 100 units/mL penicillin, and 100 μg/mL streptomycin (GIBCO-BRL)) was added and cultured at 27° C. Recombinant viruses for expressing human PepT1, which were constructed by homologous recombination, were cloned twice according to the instructions attached to the kit. A virus stock of the recombinant viruses was thus obtained.

Construction of budding-type viruses that express human PepT1 was carried out as follows. Specifically, 500 mL of Sf9 cells (2×10⁶/mL) were infected with the recombinant viruses prepared as above, so as to achieve MOI=5. After culturing at 27° C. for three days, the culture supernatant was centrifuged for 15 minutes at 800×g, and the cells and cell debris were removed. The supernatant recovered by centrifugation was centrifuged at 45,000×g for 30 minutes, and the precipitate was then suspended in PBS. The cellular components were removed by centrifuging for another 15 minutes at 800×g. The supernatant was again centrifuged at 45,000×g for 30 minutes, and the precipitate was again suspended in PBS. This suspension was the budding virus fraction. Expression of PepT1 in the virus and on the Sf-9 cell membrane was confirmed by Western analysis using anti-His antibodies. In addition, protein concentration was measured using Dc Protein Assay kit (Bio-Rad), with BSA as the standard.

2. PepT1 Functional Analysis

¹⁴C glycylsarcosine was diluted with HBSS (pH6.0) to a final concentration of 50 μM, and used as a substrate solution. 40 μL of viral solution (100 μg protein) was preincubated at 37° C. for 30 minutes. 160 μL of substrate solution that had been preheated to 37° C. was added, and the reaction was started. After one minute, 1 mL of ice-cold HBSS (pH 7.4)(hereinbelow also called “quenching solution”) was added, and the reaction was stopped. The virus-comprising reaction solution was immediately vacuum filtered using a mixed cellulose membrane filter, and washed twice with 5 mL of the quenching solution. The membrane filter was transferred to a liquid scintillation vial, 5 mL of clear-zol I was added, and the filter was dissolved. After the dissolving, a liquid scintillation counter was used to measure radioactivity on the filter. Non-specific adsorption to the filter was measured in the same way for systems where the quenching solution was added before adding the substrate solution to the viral solution, and values thus obtained were subtracted from the counts for each experiment.

The PepT1 activity of the PepT1-expressing virus with a His-tag added at its N-terminal is shown in FIG. 1. A PepT1 activity ratio of about seven times that of the wild type virus not expressing PepT1 was detected.

EXAMPLE 2

1. Preparation of PepT2-expressing Budding Baculoviruses

The full-length PepT2 gene was isolated from a human kidney library. PCR was used to integrate the gene encoding the full-length human PepT2 into pBlueBacHis2A (Invitrogen), and a full-length PepT2 transfer vector (pBlueBac) was constructed. This vector was introduced into Sf-9 cells along with the viral DNA. After cloning the recombinant virus constructed by homologous recombination, a stock with a high recombinant virus activity was constructed. Sf-9 cells were infected with the stock virus, and after culturing for a certain period, PepT2 was expressed in the virus and on the membrane of Sf-P cells. PepT2 expression in the virus and on the membrane of the Sf-9 cells was confirmed by Western analysis using anti-His antibodies. More specifically, except for using the PepT2 gene, operations were carried out according to the methods described in Example 1.

2. PepT2 Functional Analysis

³H glycylsarcosine was diluted with HBSS (pH6.0) to a final concentration of 0.8 μM, and used as a substrate solution. 40 μL of viral solution (100 μg protein) was preincubated at 37° C. for 30 minutes. 160 μL of the substrate solution preheated to 37° C. was added to commence the reaction. After one minute, 1 mL of the quenching solution was added, and the reaction was stopped. The virus-comprising reaction solution was immediately vacuum filtered using a mixed cellulose membrane filter, and washed twice with 5 mL of the quenching solution. The membrane filter was transferred to a liquid scintillation vial, 5 mL of clear-sol I was added, and the filter was dissolved. After the dissolving, a liquid scintillation counter was used to measure radioactivity on the filter. The quenching solution was added before adding the substrate solution to the viral solution, and similar manipulations were performed. Non-specific adsorption to the filter was measured and the obtained value was subtracted from the counts for each experiment.

The PepT2 activity of the PepT2-expressing virus with a His-tag added to its N-terminal is shown in FIG. 2. A PepT2 activity ratio of about nine times that of the wild type virus not expressing PepT2 was detected.

EXAMPLE 3

1. Preparation of OATP-C Expressing Baculoviruses

cDNA encoding wild type human OATP-C (OATP-C WT) was cloned as follows. Specifically, adult human liver-derived cDNA was used as a template, and the OATP-C WT cDNA was divided into two fragments and amplified using PCR with the following primer combinations:

5′ side OAHC17 primer. 5′ gat ggt acc aaa (SEQ ID NO: 1) ctg agc atc aac aac aaa aac 3′ OAHC18 primer: 5′ gat ggt acc cat (SEQ ID NO: 2) cga gaa tca gta gga gtt atc 3′ 3′ side OAHC21 primer: 5′ gat ggt acc tac (SEQ ID NO: 3) cct ggg atc tct gtt ttc taa 3′ OAHC22 primer: 5′ gat ggt acc gtt (SEQ ID NO: 4) tgg aaa cac aga agc aga agt 3′

Each of these fragments were subcloned to pT7Blue-T vector (Novagen), and clones without PCR errors were selected. Both were linked at the BglII site which exists in an overlapping region, and then cleaved at the KpnI site that exists on both ends. After incorporation at the KpnI site of pcDNA3 vector (Invitrogen), pcDNA3/OATP-C WT was obtained.

Next, with pcDNA3/OATP-C WT as a template, in vitro mutageneis using GeneEditor™ (Promega) was used to prepare cDNAs coding for OATP-C N130D in which the 130^(th) asparagine was mutated to aspartic acid, and OATP-C V174A in which the 174^(th) valine was mutated to alanine. The primers used for mutagenesis were as follows:

Primer for OATP-C N130D: 5′ gaa act aat atc gat tca tca gaa (SEQ ID NO: 5) aat 3′ Primer for OATP-C V174A: 5′ atg tgg ata tat gcg ttc atg ggt (SEQ ID NO: 6) aat 3′

The primers for use in mutagenesis and the selection primers included in kits (for bottom strand use) were both annealed to the template plasmid DNA, which had been made into a single strand. Thus, a new DNA strand was constructed. This was introduced into E. coli, and GeneEditor™ antibiotic-resistant clones were obtained. These clones were sequenced and clones containing mutations were thus selected (pcDNA3/OATP-C N130D and pcDNA3/OATP-C V174A).

Next, using pcDNA3/OATP-C WT, pcDNA3/OATP-C N130D and pcDNA3/OATP-CV174A as respective templates, PCR was carried out using the primers below, thus amplifying the respective cDNAs with SalI sites on each end.

C45 primer: 5′ gat gtc gac tta aca (SEQ ID NO 7) atg tgt ttc act 3′ C58 primer: 5′ gat gtc gac tat gga (SEQ ID NO: 8) cca aaa tca aca t 3′

These were digested with SalI, and then inserted into the SalI site of the pBlueBac His2A vector (Invitrogen). Thus transfer vectors encoding each OATP-C protein with a His-tag attached at the N-terminal were constructed (pBlueBac His2A/OATP-C WT, pBlueBac His2A/OATP-C N130D, pBlueBac His2A/OATP-C V174A).

Using Bac-N-Blue transfection kit (Invitrogen), these vectors were introduced into Sf-9 cells along with viral DNA. After five to eight days, plaque assays were used to clone the recombinant viruses in the culture supernatant. The viruses were then amplified, and a stock of highly active recombinant viruses was prepared. Sf-9 cells were infected with the stock viruses at MOI=1. After four days, recombinant viruses were recovered from the culture supernatant. OATP-C expression on the viral envelope was confirmed by Western analysis using anti-His antibodies.

2. OATP-C Functional Analysis.

³H estrone sulphate conjugate was diluted with HBSS (pH7.4) to a final concentration of 10 nM, and used as a substrate solution. 20 μL of viral solution (50 μg protein) was preincubated at 37° C. for 30 minutes. 180 μL of the substrate solution preheated to 37° C. was added, and the reaction was started. After one minute, 1 mL of ice-cold HBSS (pH7.4) (hereinafter referred to as “quenching solution”) was added, and the reaction was stopped. The virus-comprising reaction solution was immediately vacuum filtered using a mixed cellulose membrane filter, and washed twice with 5 mL of the quenching solution. The membrane filter was transferred to a liquid scintillation vial, 5 mL of clear-sol I was added, and the filter was dissolved. After the dissolving, a liquid scintillation counter was used to measure radioactivity on the filter. To measure non-specific adsorption to the filter, the reaction quenching solution was added before adding the substrate solution and similar manipulations were performed. The obtained value was subtracted from the counts for each experiment.

The activity of ³H estrone sulphate conjugate uptake is shown in FIG. 3 for three types of OATP-C-expressing viruses with His-tags added to their N-terminals. The detected ³H estrone sulphate conjugate uptake activity ratios for wild type OATP-C, N130D, and V174A were respectively 57, 41, and 36 times that of a wild type virus not expressing OATP-C. In addition, virus-derived endogenous OATP-C activity was hardly detected in experiments on the uptake in wild type viruses. Thus, it was revealed that budding baculovirus expression systems are systems with extremely low background levels. Furthermore, since OATP-C mutants (N130D, V174A) can be functionally expressed on viral envelopes, changes in substrate specificity due to SNPs can also be determined, making also applications to tailor-made therapy possible.

EXAMPLE 4 Search for PepT1 Function Inhibiting Antibodies

¹⁴C glycylsarcosine was diluted with HBSS (pH 6.0) to a final concentration of 50 μM, and used as a substrate solution. In addition, a mouse monoclonal antibody recognising the extracellular region of human PepT1 was diluted with PBS to a final concentration of 200 μg/mL, and used as an antibody solution. 20 μL (50 μg protein) of solution of budding baculoviruses expressing PepT1 with a His-tag added at the N-terminal was mixed with 20 μL of the antibody solution and incubated for one hour at 37° C. 160 μL of substrate solution preheated to 37° C. was added, and the reaction was started. After one minute, 1 mL of ice-cold HBSS (pH 7.4) (below also called “quenching solution”) was added, and the reaction was stopped. The virus-comprising reaction solution was immediately vacuum filtered using a mixed cellulose membrane filter, and washed twice with 5 mL of the quenching solution. The membrane filter was transferred to a liquid scintillation vial, 5 mL of clear-sol I was added, and the filter was dissolved. After the dissolving, a liquid scintillation counter was used to measure radioactivity on the filter. Non-specific adsorption to the filter was measured by adding the reaction quenching solution before adding the substrate solution to the viral solution, and performing similar manipulations. The obtained value was subtracted from the counts for each experiment.

The PepT1 activity inhibition by the anti-human PepT1 monoclonal antibodies is shown in FIG. 4. PepT1 activity in the absence of the antibodies was taken as the control and expressed as 100. Of the three types of anti-human PepT1 monoclonal antibodies, clone 119 inhibited PepT1 activity by about 20%, and clone 253 by about 10%, compared to the control. This PepT1 activity inhibition was statistically significant (Student t-test). Thus, budding baculovirus expression systems will be useful in the search for substrates that inhibit or promote transporter activity.

INDUSTRIAL APPLICABILITY

The present invention provides viruses that express transporters having transporter activity, and by using these viruses, transporter activity can be measured with a high sensitivity and less background level than in the past. Thus, it is expected that by employing the methods of the present invention, identification of transport substrates and driving force of transporters, and functional analysis such as kinetic analysis can be carried out more easily and accurately than before. In addition, by using such viruses, substances that inhibit or promote the transport activities of transporters expressed on the viral envelopes can be screened. Since transporters have also been reported to be involved in the transport of drugs into cells, substances that inhibit or promote the activities of transporters associated with diseases can become candidates for new pharmaceutical agents. Furthermore, by using the methods of the present invention for analysis of SNPs in transporter-encoding genes, functional changes due to transporter SNPs can be measured over a more extensive range of substrates. Application to tailor-made therapies is also possible since response to a drug can be analyzed for each individual. 

1. A method for preparing a budding virus expressing a mammalian transporter having transporter activity, the method comprising (a) culturing a host cell that (i) is infected with a budding virus that comprises a gene encoding the transporter and (ii) is recombinantly expressing the transporter; (b) expressing the transporter on the envelope of a budding virus released from the host cell; and (c) harvesting the released virus, wherein the transporter on the envelope of the released virus has transporter activity.
 2. The method of claim 1, wherein the virus is a baculovirus.
 3. The method of claim 1, wherein the transporter is a peptide transporter or an organic anion transporter.
 4. The method of claim 1, wherein the transporter is H+/di-tripeptide transporter 1 (PepT1), H+/di-tripeptide transporter 2 (PepT2), or organic anion-transporting polypeptide-C (OATP-C).
 5. The method of claim 1, wherein the host cell is an insect cell.
 6. The method of claim 5, wherein the insect cell is an Sf9 cell.
 7. The method of claim 1, further comprising assaying the harvested virus for activity of the transporter.
 8. The method of claim 1, further comprising confirming that the transporter on the envelope of the harvested virus possesses transport activity.
 9. The method of claim 1, further comprising using the harvested virus in an assay for detecting whether a test compound is transported by the transporter.
 10. The method of claim 1, wherein the budding virus of (a) is a recombinant budding virus comprising a gene encoding the transporter.
 11. A purified baculovirus, the envelope of which comprises a mammalian transporter having transporter activity.
 12. The baculovirus of claim 11, wherein the transporter is a peptide transporter or an organic anion transporter.
 13. The baculovirus of claim 12, wherein the transporter is PepT1, PepT2, or OATP-C.
 14. A method for measuring the activity of a mammalian transporter, the method comprising (a) providing a budding baculovirus the envelope of which comprises a mammalian transporter having transporter activity; and (b) measuring the activity of the mammalian transporter.
 15. The method of claim 14, wherein the transporter is a peptide transporter or an organic anion transporter.
 16. The method of claim 15, wherein the transporter is PepT1, PepT2, or OATP-C.
 17. A method of screening for a substance that inhibits or promotes transport activity of a mammalian transporter, the method comprising (a) providing a budding baculovirus the envelope of which comprises a mammalian transporter having transporter activity; (b) contacting the transporter with a test substance, and (c) selecting a substance that inhibits or promotes the transport activity.
 18. The method of claim 17, wherein the transporter is a peptide transporter or an organic anion transporter.
 19. The method of claim 18, wherein the transporter is PepT1, PepT2, or OATP-C.
 20. The method of claim 17, which comprises immobilizing the virus on a support.
 21. The method of claim 20, wherein the virus is immobilized on the support through an antibody against an envelope protein expressed on the viral envelope.
 22. The method of claim 20, wherein the virus is immobilized on the support through a biotin-streptavidin reaction by biotinylating a protein expressed on the viral envelope. 